Lithium in nature mainly exists as spodumene, lepidolite, petalite and the like in a form of pegmatite deposit, salt-lake solution and solid mineral deposit. Extraction of lithium from rock minerals has a high production cost so it has been replaced by extraction from salt-lake brine since 1990s. The brine of salt lake Atacama, salt lake Holmbreto and salt lake Uyuni in a “lithium triangle” area in triple frontier of Chili, Argentina and Bolivia, has a low content in magnesium and provides an excellent resource with low magnesium to lithium ratio. This region has been a primary manufacture location at present due to its simple and economic manufacture technique. While a characteristic of a high magnesium to lithium ratio is shown in lithium-rich lakes located in other regions around the world, such as the US saline lake, the dead sea in west Asia and the salt lake in Qaidam Basin in Qinghai province. Elements lithium and magnesium are located diagonally in the periodical table, presenting similar properties. Separating lithium from a large amount of magnesium salts is rather economically difficult. In reality, technical development of lithium salt industrialization is still in a passive and stagnated condition. Extraction of lithium from salt-lake brine with a high magnesium to lithium ratio has always been a problem unsolved until now and has drawn special attention in scientific and industrial field.
The reserve of lithium salt in salt lake of Qaidam Basin is up to 18 million tons, accounting for 22% of the total lithium reserve in the world. Lithium salt, mainly stored in surface brine, inter-crystal potential water and porous brine in the salt lakes of Qarham, Yiliping, west Taijinair, east Taijinaier and Daqaidam, has an extinguished exploration value as well as a wide application prospect. There are several ways of extracting lithium. Precipitation is suitable for preparing lithium salts from brine with a low magnesium to lithium ratio. While an economic cost is rapidly increasing as long as the mass ratio of magnesium to lithium is over 6, which is unbeneficial to separation of lithium and magnesium. Ion exchange provides solid materials with a selective adsorption on Li+. The properties of some solid materials such as MnO2 nano crystal, H1.6-xLixMn1.6O4 lithium ionic sleeve, aluminum hydroxide gel as well as solvent impregnated resins are investigated. However, their lithium adsorption capacities are not large under neutral condition, permeability of adsorbent is bad and solution loss also occurs. Calcination has been applied in preparing lithium carbonate from old brine of west Taijinair salt lake but the energy cost is high and an equipment is corroded seriously. Ionic membrane utilized in electro-osmosis is expensive and the membrane needs to be cleaned and maintained at a regular interval.
Tributyl phosphate and FeCl3 have been utilized respectively as the earliest extraction agent and the earliest co-extraction agent in solvent extraction. As the reverse extraction needs to be carried out in strong acidic condition, no successful industrial production has been reported. In Chinese invention 201210164159.8, Yuan et al., provides a modified method for extracting lithium from lithium-contained brine by multi-stage extraction, using amides and neutral phosphorus-oxygen compounds as an extraction agent, ferric chloride as a co-extraction agent and aliphatic hydrocarbon or aromatic hydrocarbon as a diluting agent. In other invention with a Chinese application no. 201410721174.7, Shidong et al., has recently provided a method for recycling lithium extraction system and for regenerating organic phase by alkali saponification. The present research group adopts ClO4− as a co-extract agent in a Chinese application No. 201210143879.6, and extraction is carried out in neutral aqueous solution. However at present, the solvent extraction only limits to liquid-liquid extraction from brine and no breakthrough changes have been achieved in solvent extraction technique.